There are two main methods for fabricating polymer dispersed liquid crystal devices, also referred to herein as PDLC devices: emulsion methods and phase separation methods. Emulsion methods have been described in U.S. Pat. Nos. 4,435,047 and 5,363,482. The liquid crystal is mixed with an aqueous solution containing binder. The liquid crystal is insoluble in the continuous phase and an oil-in-water emulsion is formed when the composition is passed through a suitable shearing device, such as a homogenizer. The emulsion is coated on a conductive surface and the water allowed to evaporate. A second conductive surface may then be placed on top of the emulsion layer by lamination, vacuum deposition, or screen printing to form a device. While the emulsion methods are straightforward to implement, droplet size distributions tend to be broad resulting in a loss in performance. For cholesteric liquid crystal devices, also referred to herein as CLC devices, this typically means reduced contrast and brightness. Phase separation methods were introduced in an effort to overcome this difficulty.
Phase separation methods have been outlined in U.S. Pat. No. 4,688,900 and in Drzaic, P. S. in Liquid Crystal Dispersions, pgs. 30–51, published by World Scientific, Singapore (1995). The liquid crystal and polymer, or precursor to the polymer, are dissolved in a common organic solvent. The composition is then coated on a conductive surface and induced to phase separate by application of ultraviolet (UV) radiation or by the application of heat or by evaporation of the solvent, resulting in droplets of liquid crystal in a solid polymer matrix. A device may then be constructed utilizing this composition. Although phase separation methods produce dispersed droplets having more uniform size distributions, there are numerous problems with this approach. For example, the long term photostability of photopolymerized systems is a concern due to the presence of photoinitiators that produce reactive free radicals. Photoinitiators not consumed by the polymerization process can continue to produce free radicals that can degrade the polymer and liquid crystals over time. Furthermore, it is also known that UV radiation is harmful to liquid crystals. Specifically, exposure to UV radiation can lead to decomposition of the chrial dopant in a cholesteric liquid crystal mixture, resulting in a change in the reflected color. The use of organic solvents may also be objectionable in certain manufacturing environments.
U.S. Pat. No. 6,423,368 proposes to overcome the problems associated with the prior art methods through the use of droplets of the liquid crystal material prepared using a limited coalescence process. In this process, the droplet-water interface is stabilized by particulate species, such as colloidal silica. Surface stabilization by particulate species such as colloidal silica is particularly preferred as it can give narrow size distribution and the size of the droplets can be controlled by the concentration of the particulate species employed. The materials prepared via this process are also referred to as Pickering Emulsions and are described more fully by Whitesides and Ross (J. Colloid Interface Sci. 169, 48 (1995)). The uniform droplets may be combined with a suitable binder and coated on a conductive surface to prepare a device. The process provides significant improvement in brightness and contrast over prior art processes. It also overcomes some of the problems associated with photoinitators and UV radiation.
U.S. patent application Ser. No. 10/718,900 shows that the maximum contrast in a bistable cholesteric liquid crystal mixture display prepared by the limited coalescence method is obtained when the uniform liquid crystal domains or droplets are coated as substantially a monolayer on the first conductive support. The bistable states in these cholesteric liquid crystal mixture displays are the planar reflecting state and the weakly scattering focal conic state. Back-scattering of light from the weakly scattering focal conic state increases drastically when there is more than a monolayer of droplets between the conductive surfaces. While the method provides displays with good brightness and contrast, there is still a need for further improvement in the appearance of the display. Specifically, there is a need to eliminate or reduce significantly the back-scattering of light at the lower end of wavelengths in the visible spectrum. Furthermore, there is a need to improve color purity in a PDLC type cholesteric liquid crystal mixture device. It is well known that the shape of the droplet causes reflections in the planar state at wavelengths lower than the main band, giving rise to a loss in color purity.
U.S. Pat. No. 6,278,505 teaches a method for improving contrast and color purity of a PDLC type cholesteric liquid crystal mixture device by adding a coloring agent to either the liquid crystal medium or to the binder. The coloring agent is chosen to absorb light in a wavelength range different from the main reflection band of the cholesteric liquid crystal mixture material. However, Okada et al. also teach fabricating a PDLC device by a phase separation process that uses UV radiation to photo-cure the binder. The process suffers from the disadvantage that numerous coloring agents, such as color dyes, are extremely sensitive to UV radiation and the presence of reactive free radicals, produced by UV irradiation.